Comes from the video game Killzone 2, Pyrrhus City is located on a hill, full of run down housing projects, apartments and slums. It pumps most of its resources to its military.
A bombardier is equivalent to a corporal in the army, which is a top ranking in militaries.
Therefore, Mos Def calling himself the bombardier from pyrrhus is like saying he’s the leader/top dog from brooklyn (giving pyrrhus conditions: slums, run down housing projects, etc.)